How to Treat Patio Black Spots

Identify correctly

Lichen, not dirt or surface mould

Treat first

Biocide kills the organism at root

Seal afterwards

Prevents regrowth for years

What causes black spots

Lichen colonies form in shaded, damp areas - especially on north-facing patios and under trees where the surface rarely dries out.

Why pressure washing fails alone

High pressure removes the visible mark but the root structure stays in the stone pores and regrows within weeks, sometimes stronger.

Biocide treatment

Applied neat to a dry patio and left 24-48 hours, biocide turns lichen brown and kills it through to the root embedded in the stone.

Gentle cleaning after treatment

Low-pressure rinse avoids damaging stone that has been weakened by lichen growth - high pressure can blow the surface off sandstone and limestone.

Sandstone and limestone

These porous stones hold lichen longer and benefit most from sealing after treatment - sealant fills the pores and blocks future colonisation.

Prevention

Improve air flow, trim back overhanging plants, sweep regularly to remove leaf litter, and re-seal every 3-5 years for permanent protection.

Frequently asked questions

Are black spots dangerous? No - they are cosmetic, but they make patios slippery when wet and can stain permanently if left untreated for years.

Can I use household bleach? Bleach only kills surface growth, does not reach roots, and can damage nearby plants and the natural colour of stone.

How long until the patio looks clean after treatment? Lichen dies within 24-48 hours and weathers off naturally over 2-4 weeks. A gentle rinse speeds up the visible improvement.

Related services and locations

Need help now, contact Jet Kings for advice and a free quote.

Need help applying this advice at your property? Call 07954 028819 or request a free quote.

What Are Patio Black Spots?

Those dark, stubborn spots on your patio that do not brush off are almost certainly lichen — a composite organism made of fungi and algae living together. Lichen attaches firmly to the stone surface and cannot be removed by ordinary sweeping or household cleaning. Unlike moss or surface algae, lichen embeds itself into the microscopic texture of the stone, which is why it persists even after rain or light pressure washing.

In Leicester and across the Midlands, black spot lichen is particularly common on north-facing patios and areas that stay damp and shaded for much of the day. Indian stone and sandstone are especially susceptible because their porous surface gives lichen a good foothold.

Why Pressure Washing Alone Does Not Work

Many homeowners assume a pressure washer will blast black spots away. It will remove the surface growth, but because lichen roots into the stone at a microscopic level, the black marks often remain. Worse, using too much pressure to try to shift them can etch the stone surface, leaving permanent damage. The right approach combines a biocide treatment to kill the lichen with gentle cleaning to remove the dead growth.

How Professionals Remove Black Spots

The professional approach to black spot removal has three stages:

Biocide treatment: A specialist cleaning solution is applied to the patio. This kills the lichen, algae and any other organic growth at the root. The treatment is left to work — it does not need to be washed off immediately.

Controlled cleaning: After the biocide has had time to work, the patio is cleaned at the correct pressure for the stone type. The dead lichen lifts away without the need for aggressive pressure.

Ongoing effect: The biocide continues working for several weeks after application, killing any remaining spores and slowing regrowth. The black spots gradually fade as the dead lichen weathers away.

Preventing Black Spots Returning

While you cannot permanently prevent lichen — it is a natural part of the outdoor environment — you can slow its return:

Keep the patio as dry and sun-exposed as possible by trimming overhanging branches and bushes

Sweep regularly to prevent debris building up in the stone texture

Consider a sealant after professional cleaning — it fills the microscopic pores where lichen anchors

Schedule a maintenance clean every 12-18 months to catch regrowth early

Frequently Asked Questions

Will bleach remove black spots from my patio?

Household bleach may lighten black spots temporarily but does not kill the lichen at the root, so the spots return. Bleach can also discolour some types of stone and harm nearby plants. Professional biocide treatments are specifically formulated for exterior stone and are both more effective and safer for your garden.

How long does it take for black spots to disappear after treatment?

The initial clean will show improvement immediately, but the full effect of the biocide treatment develops over 4-8 weeks as the dead lichen gradually weathers away. This is normal and means the treatment is working.

Can black spots damage my patio?

Lichen does not directly damage stone in the way that moss can damage roof tiles, but it does hold moisture against the surface and can make the patio more slippery when wet. The main issue is aesthetic — a patio covered in black spots looks neglected.

Is the treatment safe for my plants and grass?

Yes. We use biodegradable cleaning products that are safe for plants, grass and wildlife. We also rinse down adjacent planted areas after treatment as a precaution.